Physics Ideas

Milky Way: origin, characteristics, parts, components

The Milky Way is the barred spiral galaxy to which our solar system belongs. It comprises approximately 300 billion stars in stellar systems, as well as planets, gas and cosmic dust.

From Earth, we can see a fraction of it, as a whitish band of light that crosses the sky, very visible during the northern hemisphere summer, in the constellations of Scorpio and Sagittarius.

To the ancient Greeks, the milky appearance of this band of light was the milk spilled on the chest of Hera, wife of Zeus, god of light, sky and lightning. They called it the “Milky Way” or milk road. 

Other ancient cultures also associated the Milky Way with a path. In the Iberian Peninsula, it is known as El Camino de Santiago and, for Scandinavians, it leads to Valhalla or the abode of the gods.

Democritus, the extraordinary ancient Greek thinker, had already suggested that the Milky Way contained thousands of stars. When Galileo pointed the telescope at her, he realized that it was indeed full of stars.

Over time, the astronomers who followed him realized that the solar system was also part of the band that surrounds the night sky.

English astronomers William Herschel (1738-1822), discoverer of Uranus, along with his sister Caroline Herschel (1750-1848), created a kind of three-dimensional map of how stars are distributed in the galaxy. 

They concluded that they were arranged in an irregular disk shape, with the Sun at the center, although they could not determine their actual size. 

It wasn’t until the beginning of the 20th century that astronomers realized that the solar system was just a small fraction of a much larger group: a galaxy. And after the universe contained billions of them.

Characteristics of the Milky Way

The Milky Way is a very extensive structure. To establish distances at this level, other measurement units are required. For this reason, the literature uses:

– The light year , which is the distance that light travels in a vacuum during one year. The speed of light is constant and in a vacuum it is 300,000 km/s. Nothing in the universe moves faster.

– The parsec , pc abbreviated, is equivalent to 3.2616 light years, while one kiloparsec is 1000 parsecs or 3261.6 light years.

The shape of the Milky Way is that of a barred spiral about 60,000 pc in diameter. It is difficult to define precise boundaries as the edges are not clearly defined as the galaxy has a halo of stars and interstellar matter.

The galactic center is located towards the constellation Sagittarius, as noted by astronomer Harlow Shapley in the early 20th century, the first to estimate the size of the galactic disk.

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The solar system, in turn, is located in one of these spiral arms: the Orion arm, on the outskirts of the galaxy. Interstellar dust prevents visualization of the center, but it is possible in radio and infrared frequencies.

Thanks to them, it is known that stars revolve at high speed around a supermassive black hole, equivalent to about 3.7 million solar masses.

As for the origin of the Milky Way, cosmologists believe it is almost as old as the Big Bang, the explosion that gave rise to the entire universe.

The first stars to form galaxies must have formed about 100 million years later. That’s why cosmologists estimate its age at 13.6 billion years (the Big Bang occurred 13.8 billion years ago).

The Milky Way Era

To establish the Milky Way era, astronomers look for the oldest stars.

The age of stars is known from light, which provides information about temperature and the elements that make it up.

Stars have a nuclear reactor inside them, which needs a supply of material to function. This material is initially hydrogen, the lightest element of all, which fuses into helium. A star with a lot of hydrogen is young, and one that is poor in this element is old.

By analyzing light from a star with spectroscopic techniques, it is possible to know how much hydrogen it has, because each element absorbs certain wavelengths of light and emits others. 

The absorbed wavelengths are reflected in the spectrum as dark bands with a characteristic arrangement. This indicates an abundance of the element in question, and in this way it is possible to know if a star has a lot of hydrogen and to approximate its age.

Therefore, the Milky Way is that of its oldest stars, plus the age of its predecessors, if any. And if there were, they should only contain hydrogen, helium and lithium, the lighter elements.

The oldest stars in the Milky Way are known to be at least 13.5 billion years old, but they contain some heavy elements that could not fuse.

This means that they must have acquired them from predecessor stars, the first generation, whose lives were very short due to their large mass and exploded as supernovae. 

Adding these ages together, cosmologists estimate that the Milky Way formed 13.6 billion years ago.

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Parts of the Milky Way

The Milky Way spiral has three well-defined regions, which rotate at different speeds (the closer to the center, the faster the rotation):

– The disk , a region abundant in gas and dust approximately 40,000 pc wide and 2000 pc thick: most of the stars in the galaxy are located there, almost all of them are very hot and newly formed blue stars. 

– The lamp is a spherical thickening around the center, above and below the disk, about 6000 pc in radius. This region, unlike the disk, is sparse in dust and gas, with an ancient stellar population.

– The halo , an enormous weak sphere that surrounds the galaxy and whose center coincides with that of the disk. The stars here are grouped into globular clusters and, like the bulb, there is little interstellar material here, so the population of stars is also largely old.

spiral structure

The Milky Way is shaped like a barred spiral. Astronomers still don’t know why matter in the galaxy is organized this way. Not all spiral galaxies have bars, and many are not even spirals but elliptical.

One theory is that variations in the density of matter can propagate through space, much like the ripples in a lake when a rock is thrown. This is called the density wave theory, but it is not the only proposal to explain the presence of spiral arms.


satellite galaxies

There are several smaller galaxies that accompany the Milky Way, the best known being the Magellanic Clouds.

The Sagittarius dwarf galaxy and one more have recently been found, which scientists have yet to agree on whether it is a satellite galaxy per se or part of the Milky Way – the Great Dog dwarf galaxy.

There may even be other Milky Way satellite galaxies that cannot be seen from our location in one of the spiral arms. The Milky Way’s powerful gravity draws them in and, in millions of years, they will become part of it.

central black hole

Thanks to infrared telescopes, astronomers were able to track the movement of stars near the center of the galaxy.

There is an intense X-ray source known as Sgr A (Saggitarius A), which is believed to be the supermassive black hole that all galaxies, including our own, have at their center.

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A black hole is estimated to have about 4 million solar masses. From it emanates a glow produced by interstellar matter that flows continuously. Occasionally, a violent look indicates that a star has ended up inside it.


The Milky Way’s splendor is due to the stars that populate it: between 200 and 400 million. Our Sun is an ordinary mid-life star, located in the arm of Orion, 7900 cp from the bustling galactic center.

There are many types of stars, classified according to their mass and temperature. They are also classified according to the content of light elements, hydrogen and helium, or heavier elements, which astronomers often call metals.

The latter are younger stars, called Population I, while the former are older and are known as Population II.

In galaxies like the Milky Way, there are stars from both populations. In the spiral arms and in the galactic disk, population II prevails, while in the halo and bulb those of population I prevail.


Until relatively recently, the only star system known to have planets was the Solar System. In it there are two types of planets; rocky ones like Earth and giants like Jupiter.

From the 1990s onwards, planets were discovered in other star systems: extrasolar planets or exoplanets.

So far, there are more than 3,000 discoveries and the number doesn’t stop. The vast majority are planets of the Jovian type, that is, gas giants, but some have been found like Earth.

interstellar matter

The space between stars is filled with interstellar gas and dust. When the Milky Way is observed from Earth, darker lines and areas are seen, where gas and dust abound. It consists mainly of light elements: hydrogen and helium, with traces of heavier elements.

Interstellar matter plays a fundamental role in the galaxy and the universe, as it is the raw material of stars and planetary systems.

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